Integrated circuit that provides access to an output node of a filter having an adjustable bandwidth

Abstract

An IC includes an externally accessible terminal, a filter, and an output stage. The filter has an adjustable bandwidth and has an output node coupled to the externally accessible terminal, and the output stage has an input node coupled to the output node of the filter. By providing access to a node that is between the filter and an amplitude-limiting output stage, the IC allows one to adjust the bandwidth of the filter—or the bandwidth of a circuit, such as an amplifier, that includes the filter—by measuring the amplitude of a signal that can have a relatively high S/N ratio without being clipped by the output stage. Consequently, such access can eliminate the need for a high-precision bandwidth-adjust setup and its long bandwidth-adjust times.

1. An integrated circuit, comprising:
 first and second externally accessible terminals;  
 a filter having an adjustable bandwidth and having a first output node coupled to the first externally accessible terminal; and  
 an output stage having a first input node coupled to the first output node of the filter and having a first output node coupled to the second externally accessible terminal.  
 
     
     
       2. The integrated circuit of  claim 1 , further comprising a gain stage disposed between the filter and the first externally accessible terminal, the gain stage having an input node coupled to the first output node of the filter and having an output node coupled to the first externally accessible terminal. 
     
     
       3. The integrated circuit of  claim 1 , further comprising a peak detector disposed between the filter and the first externally accessible terminal, the peak detector having an input node coupled to the first output node of the filter and having an output node coupled to the first externally accessible terminal. 
     
     
       4. The integrated circuit of  claim 1 , further comprising an analog-to-digital converter disposed between the filter and the first externally accessible terminal, the analog-to-digital converter having an input node coupled to the first output node of the filter and having an output node coupled to the first externally accessible terminal. 
     
     
       5. The integrated circuit of  claim 1  wherein the output stage comprises an
 amplitude-limiting output stage.  
 
     
     
       6. The integrated circuit of  claim 1 , further comprising:
 a package; and  
 wherein the first and second externally accessible terminals each have a respective portion disposed outside of the package.  
 
     
     
       7. The integrated circuit of  claim 1 , further comprising:
 third and fourth externally accessible terminals;  
 wherein the filter has a second output node coupled to the third externally accessible terminal; and  
 wherein the output stage has a second input node coupled to the second output node of the filter and has a second output node coupled to the fourth externally accessible terminal.  
 
     
     
       8. An integrated circuit, comprising:
 a filter having an adjustable bandwidth and operable to generate a filtered signal having an amplitude; and  
 a peak detector coupled to the filter and operable to provide the amplitude of the filtered signal to an apparatus that is external to the integrated circuit.  
 
     
     
       9. The integrated circuit of  claim 8  wherein the peak detector is operable to provide the amplitude of the filtered signal to an apparatus that is operable to adjust the bandwidth of the filter based on the amplitude of the filtered signal. 
     
     
       10. The integrated circuit of  claim 8 , further comprising:
 an externally accessible terminal; and  
 an output stage coupled to the filter and operable to limit the amplitude of the filtered signal and to provide the amplitude-limited signal on the externally accessible terminal.  
 
     
     
       11. The integrated circuit of  claim 8 , further comprising:
 a gain stage coupled between the filter and the peak detector and operable to amplify the filtered signal to generate an amplified signal having an amplitude; and  
 wherein the peak detector is operable to provide the amplitude of the amplified signal to the apparatus.  
 
     
     
       12. The integrated circuit of  claim 8 , further comprising an analog-to-digital converter coupled to the peak detector and operable to convert the amplitude of the filtered signal into a digital value and to provide the digital value to the apparatus. 
     
     
       13. The integrated circuit of  claim 8  wherein:
 the signal comprises a differential signal; and  
 the filtered signal comprises a differential filtered signal.  
 
     
     
       14. A fiber-optic receiver, comprising:
 first and second externally accessible terminals;  
 a device operable to generate a first electrical signal from a received optical signal; and  
 a first amplifier coupled to the device and comprising, 
 a filter having an adjustable bandwidth and having an output node coupled to the first externally accessible terminal, the filter operable to filter the first electrical signal to generate on the output node a filtered electrical signal having an amplitude, and  
 an output stage having an input node coupled to the output node of the filter and having an output node coupled to the second externally accessible terminal, the output stage operable to limit the amplitude of the filtered first electrical signal.  
 
 
     
     
       15. The fiber-optic receiver of  claim 14  wherein the device comprises a photo-diode. 
     
     
       16. The fiber-optic receiver of  claim 14 , further comprising a preamplifier coupled between the device and the amplifier and operable to preamplify the electrical signal. 
     
     
       17. The fiber-optic receiver of  claim 14  wherein the amplifier further comprises a gain stage disposed between the filter and the first externally accessible terminal, the gain stage having an input node coupled to the first output node of the filter and having an output node coupled to the first externally accessible terminal. 
     
     
       18. The fiber-optic receiver of  claim 14  wherein the amplifier further comprises a peak detector disposed between the filter and the first externally accessible terminal, the peak detector having an input node coupled to the first output node of the filter and having an output node coupled to the first externally accessible terminal. 
     
     
       19. The fiber-optic receiver of  claim 14  wherein the amplifier further comprises:
 an analog-to-digital converter disposed between the filter and the first externally accessible terminal, the analog-to-digital converter having an input node coupled to the first output node of the filter and having output nodes; and  
 a parallel-to-serial interface disposed between the converter and the first externally accessible terminal, the interface having input nodes respectively coupled to the output nodes of the converter and having an output node coupled to the first externally accessible terminal.  
 
     
     
       20. The fiber-optic receiver of  claim 14 , further comprising:
 third and fourth externally accessible terminals; and  
 a second amplifier coupled to the first amplifier and comprising, 
 a filter having an adjustable bandwidth and having an output node coupled to the third externally accessible terminal, the filter operable to filter the amplitude-limited first electrical signal to generate on the output node a filtered second electrical signal having an amplitude, and  
 an output stage having an input node coupled to the output node of the filter and having an output node coupled to the fourth externally accessible terminal, the output stage operable to limit the amplitude of the filtered second electrical signal.  
 
 
     
     
       21. A fiber-optic receiver, comprising:
 a device operable to generate an electrical signal from a received optical signal; and  
 an amplifier coupled to the device and comprising, 
 a filter having a adjustable bandwidth and operable to filter the electrical signal to generate a filtered electrical signal having an amplitude, and  
 a peak detector coupled to the filter and operable to provide the amplitude of the filtered electrical signal to an apparatus that is operable to adjust the bandwidth of the filter based on the amplitude of the filtered electrical signal.  
 
 
     
     
       22. A method, comprising:
 filtering a signal having a frequency with a filter that forms part of an amplifier disposed on an integrated circuit; and  
 measuring the filtered signal at an intermediate node of the amplifier.  
 
     
     
       23. The method of  claim 22 , further comprising:
 wherein measuring the filtered signal comprises measuring the amplitude of the filtered signal; and  
 adjusting the bandwidth of the filter so that the filtered signal has a predetermined amplitude when the signal has the frequency.  
 
     
     
       24. The method of  claim 22 , further comprising:
 amplifying the filtered signal; and  
 wherein measuring the filtered signal comprises measuring the amplitude of the amplified filtered signal.  
 
     
     
       25. The method of  claim 22  wherein measuring the filtered signal comprises measuring the amplitude of the filtered signal on a terminal that is coupled to the intermediate node of the amplifier and that is accessible from the outside of the integrated circuit. 
     
     
       26. A method, comprising:
 filtering a first signal having a first frequency with a filter disposed on an integrated circuit;  
 measuring the amplitude of the filtered first signal at a node that is between the filter and an output stage disposed on the integrated circuit;  
 filtering a second signal having a second frequency with the filter;  
 measuring the amplitude of the filtered second signal at the node; and  
 adjusting the bandwidth of the filter so that the amplitude of the filtered second signal has a predetermined relationship to the amplitude of the filtered first signal.  
 
     
     
       27. The method of  claim 26  wherein:
 the second frequency is higher than the first frequency; and  
 adjusting the bandwidth of the filter comprises adjusting the bandwidth of the filter so that the amplitude of the filtered second signal is or is approximately 3 dB down from the amplitude of the filtered first signal.